Reprinted  from  The  Journal  of  Geology,  Vol.  II,  No.  7, 
October  - November,  1 894. 


On  a Basic  Rock  Derived  From 

Granite 


By  C.  H.  SMYTH,  Jr. 


CHICAGO 

&!)e  atntbecstfg  of  attfcago  ©mss 

1894 


ON  A BASIC  ROCK  DERIVED  FROM  GRANITE. 


While  studying  the  hematite  deposits  of  St.  Lawrence  and 
Jefferson  counties,  New  York,  under  the  direction  of  Dr.  James 
Hall,  State  Geologist,  the  writer  had  occasion  to  examine  some 
interesting  rocks  associated  with  the  ores,  which  are  worthy  of 
note. 

A brief  description  of  the  rocks  is  given  in  the  report  on  the 
region  presented  to  Dr.  Hall,  in  which  especial  attention  is  paid 
to  their  relation  to  the  problem  of  the  origin  of  the  iron  ores. 
The  aim  of  the  present  paper  is  to  supplement  that  report  with 
a more  complete  account  of  one  phase  of  these  rocks,  occurring 
at  the  Old  Sterling  mine,  in  Antwerp,  Jefferson  county. 

Mode  of  occurrence  of  the  rock. — The  mine  consists  of  a large 
open  pit,  together  with  considerable  underground  workings. 
The  surface  rock  is  Potsdam  sandstone,  beneath  which  the  ore, 
a red  hematite,  occurs  in  large  irregular  masses,  intimately  asso- 
ciated with  a rock  of  entirely  different  character,  which  is  the 
subject  of  this  paper.  The  contact  between  this  rock  and  the 
ore  is  extremely  irregular,  as  is  well  shown  in  the  open  pit  where 
the  ore  has  been  removed  leaving  the  rock  in  projecting  knobs, 
ridges,  and  walls,  with  intervening  pockets  and  hollows.  In  the 
underground  workings  it  is  not  uncommon  for  the  ore  to  be  sud- 
denly cut  off  by  the  rock  and  to  appear  again  after  passing 
through  a greater  or  less  thickness  of  it.  The  contact  is  evidently 
irruptive  and  is  precisely  like  the  contacts  of  granite  and  limestone 
which  are  common  in  the  region,  although  the  rocks  involved  have 
a very  different  appearance.  Emmons,1  who  has  given  the  only 
detailed  account  of  the  ore  and  associated  rock,  considered 
them  both  igneous,  and  called  the  rock  serpentine,  a name 
which  has  clung  to  it  ever  since.  The  facts  adduced  by  Emmons 

1 Geology  of  New  York.  Second  District,  p.  97, 

667 


668 


THE  JOURNAL  OF  GEOLOGY. 


to  prove  the  igneous  nature  of  the  rocks  were  hardly  of  a char 
acter  to  justify  the  conclusion,  and  that  he  was  right  in  regard 
to  the  so-called  serpentine  must  be  regarded  as  a mere  matter  of 
chance. 

Descriptioji  of  the  rock. — A hasty  examination  of  many  speci- 
mens of  the  rock  would  seem  to  warrant  Emmons’  determination 
of  it  as  serpentine.  It  is  dark  green  or  black,  massive,  and  very 
fine  grained  or  quite  aphanitic,  with  rather  waxy  lustre,  and 
abundantly  slickensided.  Thus  far  it  is  precisely  like  serpen- 
tine, and  many  specimens  show  no  other  prominent  features. 
But  most  of  the  rock  is  mottled  with  abundant  white,  vitreous 
spots,  of  extremely  variable  size.  Where  these  are  large  enough 
to  be  clearly  seen  (and  they  may  reach  three  or  four  inches  in 
diameter), it  is  evident  that  they  are  fragments  of  quartz.  Speci- 
mens in  which  the  grains  are  small  and  evenly  distributed  closely 
resemble  a porphyritic  rock  with  glassy  or  aphanitic  groundmass. 
In  the  presence  of  this  quartz,  there  is  a marked  divergence 
from  the  ordinary  character  of  serpentine. 

The  slickensides  which  are  so  abundant  in  hand  specimens 
are  developed  on  a large  scale  in  the  rock  exposed  in  the  mine 
pit.  They  run  in  every  direction,  and  on  this  account  most  of 
the  exposed  surfaces  are  slickensided,  being,  as  a rule,  curved 
and  showing  a beautiful  polish.  Such  surfaces  measuring  more 
than  one  hundred  square  feet  are  not  uncommon.  A good 
example  appears  in  the  centre  of  the  pit  upon  a dome-like  mass 
of  rock. 

Origin  of  the  rock. — Accepting  the  evidence  afforded  by  the 
contact,  that  the  rock  must  be  igneous,  a difficulty  is  met  at  once 
in  its  apparently  contradictory  features.  Its  serpentinous 
aspect  suggests  derivation  from  a basic  rock,  but  conflicting 
with  such  a supposition  is  the  presence  of  much  quartz.  This 
mineral  is,  moreover,  so  distributed  that  it  cannot  be  accounted 
for  as  inclusions,  or,  to  any  great  extent,  as  secondary.  These 
facts,  noted  in  the  field,  led  the  writer  to  a close  examination  of 
the  exposures  in  the  hope  of  finding  some  portion  of  the  rock 
which,  through  less  complete  alteration,  would  give  some  clue  to 


ON  A BASIC  ROCK  DERIVED  FROM  GRANITE.  669 


its  origin.  The  examination  resulted  in  the  finding  of  several 
small  patches,  which,  though  considerably  altered,  still  retain 
enough  of  their  original  character  to  show  conclusively  that  they 
are  granite.  The  transition  between  the  fairly  fresh  granite 
and  the  serpentine-like  rock  is  very  gradual,  the  feldspar  los- 
ing its  fresh  appearance  and  passing  over  into  a green  aggregate, 
while  the  quartz  decreases  in  quantity,  till  in  some  specimens  it 
is  wholly  lacking.  To  the  naked  eye  no  other  minerals  are 
visible,  the  original  granite  having  been  a coarse,  pegmatitic 
variety. 

The  alteration  of  an  acid  granite  into  a nearly  black,  appar- 
ently basic,  rock  is  an  exceptional  phenomenon,  and  yet  in  this 
instance  the  field  evidence  alone  is  of  such  conclusive  nature 
that  there  can  be  no  doubt  that  such  is  the  case.  Furthermore, 
the  operation  has  taken  place  on  a large  scale,  for  there  are  hun- 
dreds of  tons  of  the  altered  rock  in  sight,  and  it  is  impossible  to 
tell  how  much  is  underground. 

Additional  evidence  that  the  rock  is  an  alteration  of  granite 
is  afforded  by  the  fact  that  at  the  bottom  of  the  underground 
workings  the  ore  rests  upon  a granite  which  differs  only  in 
freshness  from  the  material  of  the  least  altered  patches  in  the 
serpentine.  But  even  this  granite  shows  the  beginnings  of  the 
alteration. 

Megascopical  aspect  of  the  alteration. — A specimen  of  granite 
from  the  bottom  of  the  shaft  is  a decidedly  coarse-grained  rock, 
made  up  of  quartz  and  feldspar,  the  latter  predominating. 
Besides  these  distinct  minerals  there  is  a limited  amount  of 
greenish  material  which  fills  the  interstices  between  the  essential 
minerals.  The  latter  have  very  irregular  outlines,  the  feldspar 
occurring  in  decidedly  larger  individuals  than  the  quartz.  The 
former  mineral  generally  has  bright  cleavage  faces ; less  often 
they  are  dull  and  earthy.  The  color  is  gray  with  a very  faint 
pink  tinge.  The  quartz  is  colorless  and  clear,  with  the  usual 
vitreous  lustre. 

The  granite  of  the  least  altered  patches  in  the  “serpentine” 
differs  from  such  a specimen  in  having  a larger  amount  of  the 


670 


THE  JOURNAL  OF  GEOLOGY. 


green  aggregate,  and  in  the  color  of-  the  feldspar.  This  color  is 
a decided  pink  or  red,  but  there  can  be  no  doubt  that  it  is  itself 
a result  of  alteration,  having  replaced  an  original  gray  like  that 
of  the  feldspar  of  the  specimens  from  greater  depth.  These 
patches  of  comparatively  little  altered  granite  are  only  a few 
feet  in  diameter,  and  shade  off  into  the  typical  serpentine-like 
rock  on  all  sides.  The  passage  is  gradual  as  a whole,  but  more 
rapid  in  some  spots  than  in  others,  so  that,  while  it  is  impossible 
to  draw  any  line  of  demarcation  in  the  stages  of  the  process, 
lines  uniting  equally  altered  portions  around  a granite  core  would 
be  extremely  irregular. 

From  the  centre  of  such  a core  outward  the  green  aggregate 
increases,  while  the  feldspar  gradually  disappears,  till  finally 
there  remains  a waxy,  deep  green  mass  holding  fragments  of 
quartz.  In  some  highly  altered  specimens  the  quartz  is  very 
conspicuous  against  the  dark  groundmass ; in  others  it  is 
entirely  absent,  though  this  is  never  true  of  large  masses.  In 
still  other  cases  lumps  of  quartz  several  inches  in  diameter 
occur.  These  always  lie  close  to  one  another  and  usually  along 
definite  zones,  showing  clearly  that  they  are  fragments  of  crushed 
veins. 

In  the  slightly  altered  granite  cores  there  are  no  conspicuous 
indications  of  disturbances,  the  slickensides  being  confined  to 
the  highly  altered  phases  of  the  rock,  in  which,  as  stated  above, 
they  are  very  prominent.  This  fact,  together  with  the  great 
irregularity  in  the  direction  of  the  slickensides,  suggests  that  the 
movements  which  have  formed  the  polished  surfaces  may  have 
resulted  from  changes  of  bulk  in  the  rock  attendant  upon  the 
alteration,  as  in  the  case  of  true  serpentines.1 

It  should  be  noted,  however,  that  the  granite  cores  are  so 
small  that  they  might  fail  to  give  evidence  of  considerable 
movements  in  the  mass  as  a whole,  and  such  movements  may 
account  for  the  slickensides,  as  they  do  for  the  crushing  of  the 
quartz  in  veins  and  scattered  through  the  rock.  That  the  latter 

1 J.  S.  Diller,  Geology  of  the  Lassen  Peak  District,  8th  Ann.  Rept.,  U.  S.  G.  S‘‘ 

p.  401. 


ON  A BASIC  ROCK  DERIVED  FROM  GRANITE.  67 1 

is  the  case  is  clearly  shown  by  the  microscopic  structure  of  the 
rock,  as  stated  below. 

Microscopical  details  of  the  alteration. — A microscopical  exam- 
ination of  sections  illustrating  all  of  the  phases  of  alteration 
shows  some  variation  in  the  intermediate  stages  of  the  process, 
but  considerable  uniformity  in  the  final  results. 

Sections  of  the  least  altered  granite  show  it  to  consist  of 
orthoclase,  microcline,  and  quartz,  with  occasional  stout  prisms 
of  apatite  and  irregular  masses  of  tourmaline.  Granite  of  this 
character  is  quite  common  in  the  region,  and  has  been  previously 
described  by  the  writer.1  The  feldspar  has  the  common  dull  and 
cloudy  appearance.  The  quartz  contains  abundant  fluid  inclusions 
and  great  numbers  of  the  hair-like  bodies  usually  considered 
rutile.  Undulatory  extinction  is  constant,  and  much  granulation 
is  shown  in  nearly  all  sections. 

Alteration  begins  with  the  development  of  a greenish  aggre- 
gate in  the  feldspar.  This  may  form  irregular  masses,  or  may 
be  confined  to  cracks.  Very  often  quite  large  portions  of  the 
aggregate  have  angular  outlines  formed  by  cleavage  cracks  of 
the  feldspar.  Where  the  aggregate  has  formed  in  a zone  of 
crushing,  it  usually  contains  small,  angular  fragments  of  feldspar 
which  at  first  sight  look  like  crystals  of  some  newly  formed 
mineral.  The  areas  of  the  aggregate  are  very  unequally  distrib- 
uted in  the  granite,  one  portion  of  a specimen  being  greatly 
changed,  while  another  portion  remains  unaltered.  In  some 
cases  this  is  clearly  due  to  the  arrangement  of  cracks  and 
crushed  zones,  but  often  there  is  no  apparent  reason  for  it.  As 
the  alteration  proceeds  the  areas  of  the  aggregate  gradually 
extend  until  they  entirely  replace  the  feldspar,  leaving  no  trace 
of  its  former  presence. 

At  the  same  time  the  quartz  is  attacked,  but,  as  a rule,  it 
yields  much  more  slowly  than  the  feldspar.  In  the  absence  of 
cleavage  the  alteration  proceeds  along  the  borders  of  the  grains 
and  in  the  irregular  cracks.  The  areas  of  alteration  product 

1 C.  H.  Smyth,  Jr.:  Petrography  of  the  Gneisses  of  the  Town  of  Gouverneur, 
N.  Y,  Trans.  N.  Y.  Acad.  Sci.,  XII.,  p.  210. 


672 


THE  JOURNAL  OF  GEOLOGY. 


thus  formed  never  have  the  sharp,  angular  outline  that  they  have 
in  the  case  of  the  feldspar.  Irregular  tongues  of  the  aggregate 
eat  their  way  into  the  quartz,  gradually  spreading,  cutting  across, 
and  separating  grains  originally  continuous,  and  finally  entirely 
replacing  them.  Such  complete  replacement  of  the  quartz  is, 
however,  exceptional,  and  seldom  extends  through  any  consider- 
able mass  of  the  rock.  Although  the  quartz  usually  lags  behind 
the  feldspar  in  the  process  of  alteration,  the  degree  of  change  in 
the  two  minerals  has  no  constant  relation.  For,  on  the  one 
hand,  a section  whose  feldspar  is  completely  replaced  by  the 
aggregate  may  retain  most  of  its  original  quartz,  while,  on  the 
other  hand,  a section  with  quite  fresh  feldspar  may  show  much 
alteration  in  the  quartz.  Not  uncommonly  the  alteration  pro- 
ceeds most  rapidly  along  the  contact  between  the  quartz  and 
feldspar.  It  is  hardly  probable  that  this  results  from  chemical 
causes  ; it  must,  rather,  be  due  to  the  more  ready  circulation  of 
solutions  along  these  contacts.  This  may,  perhaps,  be  accounted 
for  by  a tendency  for  quartz  and  feldspar  to  separate  under 
mechanical  strains. 

The  extreme  result  of  the  alteration  is  a mass  of  the  greenish 
aggregate  with  no  trace  of  either  quartz  or  feldspar.  But  more 
commonly  the  rock  consists  of  the  green  aggregate  with  a 
greater  or  less  number  of  quartz  grains. 

Under  low  powers  the  aggregate  has  a felt-like  appearance,  and 
a green  or  yellowish  color.  With  crossed  nicols  it  shows  aggre- 
gate polarization  of  varying  intensity,  the  most  thoroughly  altered 
sections  being  nearly  isotropic. 

With  higher  powers  the  aggregate  is  seen  to  be  made  up  of 
small,  irregular  scales,  with  a single  pronounced  cleavage. 
These  scales  are  quite  pleochroic  in  green  and  yellow,  have  a 
parallel  extinction,  and  low  double  refraction.  From  these  facts 
it  is  very  probable  that  the  scales  consist  of  some  member  of  the 
chlorite  group,  or  of  one  of  the  nearly  related  hydrous  silicates. 
An  absolute  determination  of  species  is  out  of  the  question,  and 
it  is,  moreover,  probable  that  more  than  one  species  enter  into 
the  composition  of  the  aggregate.  Some  sections  show  mingled 


ON  A BASIC  ROCK  DERIVED  FROM  GRANITE. 


673 


with,  or  replacing,  the  green  scales,  colorless  scales  with  similar 
cleavage,  but  no  pleochroism,  and  having  strong  double  refrac- 
tion. In  this  case  the  mineral  is  probably  muscovite.  It  is 
much  less  abundant  than  the  chloritic  mineral  and  disappears  as 
the  alteration  becomes  more  complete.  By  this  it  is  not  meant 
to  imply  that  the  muscovite  is  an  essential  step  in  the  process  of 
change,  as  in  most  cases  there  is  no  trace  of  it,  even  in  the  earlier 
stages  of  alteration.  Other  substances  are  present  in  minor 
quantity,  and  generally  of  undeterminable  character.  In  many 
cases  they  are  evidently  the  result  of  the  alteration  of  the  normal 
green  aggregate  by  ordinary  surface  agents,  with  the  production 
of  iron  oxide,  carbonates,  etc.  Such  weathering  often  bring 
out  very  clearly  a wavelike  banding  in  the  sections,  which  very 
closely  resembles  flow  structure.  When  this  structure  appears 
in  a section  of  the  most  highly  altered  rock,  composed  of  the 
very  low,  doubly  refracting  aggregate,  the  likeness  to  a section  of 
a glassy  volcanic  rock  is  striking. 

Cataclastic  structure  is  very  pronounced  in  most  sections, 
when  the  alteration  has  not  proceeded  so  far  as  to  hide  it,  and 
there  can  be  no  doubt  that  the  crushing  played  an  important 
part  in  the  process  of  change. 

Chemistry  of  the  process. — As  microscopical  study  gives  no  defi- 
nite information  in  regard  to  the  chemical  composition  of  the 
altered  granite,  an  analysis  has  been  made  of  a carefully  selected 
sample.  For  this  purpose  a specimen  was  chosen  representing 
the  extreme  result  of  the  process  of  alteration,  being  nearly  free 
from  quartz,  with  a deep  green  color  and  waxy  lustre.  A thin 
section  cut  from  the  specimen  shows  a mingling  of  green  and 
yellow  aggregates,  with  no  trace  of  feldspar  or  quartz.  The 
results  of  the  analysis  are  shown  in  column  I.  No  analysis  has 
been  made  of  the  fresher  granite,  because  even  the  best  speci- 
mens are  so  much  altered  that  the  results  obtained  would 
give  no  clearer  idea  of  the  original  composition  of  the  rock  than 
can  be  gathered  from  a consideration  of  its  mineralogical  com- 
position. 


674  the  journal  of  geology. 


I. 

II. 

III. 

IV. 

Si02 

- 29.70 

26.88 

29.45 

46.90 

A1203  - 

17.03 

17.52 

18.25 

35-73 

Fe203 

— 1 

— 

8.17 

I 

FeO  - - 

27.15 

29.76 

15.12 

2.48 

MgO 

10.66 

13.84 

15-32 

0.83 

CaO  - 

1.68 

— 

0.45 

0.45 

Na20 

0.56 

— 

— 

0.48 

K20  - 

O.IO 

— 

— 

6.41 

h2o 

- 11.79 

n-33 

12.57 

5.00 

98.63 

99-33 

99-33 

98.88 

I.  Greatly  altered  granite,  Old  Sterling  mine. 

II.  Prochlorite,  St.  Christophe.3 

III.  Delessite,  Zwickau.3 

IV.  Alteration  product  of  doubtful  origin  (probably  derived  from  granite), 

Caledonia  mine. 

It  has  been  shown  that  the  original  rock  was  an  acid  granite, 
consisting  almost  wholly  of  orthoclase,  microcline  and  quartz, 
with  no  ferromagnesian  constituents.  From  this  there  can  be  no 
doubt  that  it  contained  not  less  than  70  per  cent,  of  silica,  with  a 
large  content  of  alumina  and  alkalies,  and  little  iron,  magnesia, 
and  lime.  The  analysis  shows  that  the  process  of  alteration  con- 
sisted of  a decided  decrease  in  the  percentage  of  silica  and 
alkalies,  with  an  equally  marked  increase  of  iron  and  magnesia, 
and  the  addition  of  much  water.  Moreover,  this  has  not  been  a 
mere  removal  of  some  constituents,  leaving  relatively  increased 
proportions  of  the  others,  but,  on  the  contrary,  there  has  been  an 
actual  addition  of  material  from  a foreign  source.  It  is  hardly 
necessary  to  say  that  the  composition  of  this  alteration  product 
is  totally  unlike  that  of  the  product  that  would  result  from  the 
alteration  of  such  a granite  under  ordinary  conditions.  Of 
course  the  analysis  represents,  as  already  stated,  the  extreme 
result  of  alteration,  but  it  is  not  probable  that  the  results  would 

xThe  iron  in  I.  and  IV.  is  all  calculated  as  ferrous,  but  there  can  be  no  doubt 
that  some  of  it  is  in  the  ferric  condition. 


2 Dana’s  System  of  Mineralogy,  p.  654. 
3 Ibid.,  p.  660. 


ON  A BASIC  ROCK  DERIVED  FROM  GRANITE.  675 

be  very  different  for  an  average  sample.  The  silica  uldoM  be 
increased  by  the  grains  of  quartz,  but  would  hardly  exceed  40  to 
50  per  cent. 

The  results  of  the  analysis  and  of  the  microscopical  study 
bring  into  question  the  propriety  of  applying  to  the  rock  the 
name  serpentine.  Modern  usage  seems  to  justify  the  use  of  the 
designation  “serpentine”  for  rocks  composed  largely  of  the  mineral 
serpentine.  But  the  analysis  of  the  green  aggregate  composing 
the  larger  part  of  the  rock  under  discussion  shows  a composition 
so  different  from  that  of  serpentine,  that  there  can  be  no  doubt 
of  the  impropriety  of  applying  this  name  to  the  rock.  Analyses 
of  different  portions  of  the  green  aggregate  would  probably 
yield  decidedly  different  results,  so  that  it  seems  useless  to 
attempt  to  identify  it  as  a whole  with  any  mineral  species  ; but 
the  analysis  given,  as  well  as  the  optical  properties,  indicates 
a much  closer  relationship  with  the  chlorites  than  with  serpen- 
tine. This  relationship  is  illustrated  by  analysis  II.  and  III., 
which  are  very  similar  to  I.,  the  difference  being  no  greater  than 
would  naturally  result  from  the  variability  in  the  composition  of 
the  minerals  concerned. 

Cause  of  the  alteration. — In  endeavoring  to  ascertain  the  cause 
which  has  led  to  such  a complete  change  in  the  granite,  it  is  evi- 
dent that  search  must  be  made  among  the  class  of  processes 
to  which  Roth1  gives  the  name  of  “ complicated  weathering.” 
For  the  alteration  is  not  of  a kind  that  could  be  produced  by 
the  simple  agents  of  the  normal  weathering  of  rocks,  nor  do  the 
facts  indicate  that  it  is  aresult  of  ordinary  dynamic.metamorphism, 
though,  as  stated  above,  crushing  of  the  rock  has  been  an  impor- 
tant factor.  The  granite  must  have  been  attacked  by  some  pow- 
erful chemical  agent,  whose  action  was  not  general,  but,  on  the 
contrary,  limited  to  this  particular  locality,  and  to  such  others  as 
show  analogous  alteration  products.  A clue  to  the  nature 
of  the  agent  is  afforded  by  the  composition  of  the  altera- 
tion product,  and  by  the  character  of  the  associated  rocks.  From 
the  analysis  it  is  clear  that  the  alteration  has  been  brought  about 

IJ.  Roth,  Allgemeine  und  Chemische  Geologie,  Vol.  I.,  p.  2. 


6y6 


THE  JOURNAL  OF  GEOLOGY. 


by  the  removal  of  certain  constituents  of  the  granite,  and  the 
addition  of  a great  amount  of  iron,  with  less  magnesia  and  much 
water.  The  association  of  the  altered  granite  with  iron  ore,  sug- 
gests that  the  latter  is  in  some  way  connected  with  the  process 
of  alteration.  That  such  a connection  does  exist  is  shown  by 
the  fact  that  rocks  similar  to  the  altered  granite,  and  sometimes 
of  like  origin,  occur  at  all  of  the  ore  mines  of  this  vicinity ; 
while,  on  the  other  hand,  nothing  of  the  kind  is  found  away 
from  the  ore,  although  granite  is  a very  common  rock. 

Accepting  the  connection  between  the  alteration  of  the 
granite  and  the  presence  of  the  iron  ores,  two  hypotheses  are 
suggested  to  account  for  the  phenomena.  The  discussion  of 
these  two  hypotheses  involves  the  whole  question  of  the  origin 
of  the  iron  ores,  which  is  considered  at  some  length  in  the 
report  to  which  reference  has  been  made.  For  present  pur 
poses  only  a very  brief  summary  of  the  most  salient  points  is 
necessary. 

According  to  one  hypothesis  the  granite  is  younger  than,  and 
has  been  intruded  into,  the  iron  ore.  As  a result  of  this  intru- 
sion the  granite  has  undergone  marked  endomorphic  changes, 
during,  and  subsequent  to,  the  intrusion,  becoming  heavily 
charged  with  iron  and  assuming  its  present  form.  But  this  hypoth- 
esis is  rendered  doubtful  (aside  from  its  inherent  weakness) 
by  the  absence  of  any  contact  phenomena  in  the  iron  ore,  and 
by  the  presence  of  an  analogous  serpentine-like  rock  at  another 
ore  mine,  which  is  derived  from  a finely  laminated  gneiss,  instead 
of  a granite.  In  fact,  there  is  nothing  to  indicate  that  the  change 
in  the  granite  associated  with  iron  ore  is,  in  any  way,  the  result 
of  the  action  of  heated  solutions  generated  at  the  time  of 
intrusion,  while  much  evidence  is  at  hand  to  prove  that  this  is 
not  the  case.  There  is  more  probability  in  a modification  of  this 
hypothesis,  by  which  it  is  assumed  that  the  ore  was  originally 
siderite  and  that  oxidizing  meteoric  waters  changed  it  to  the 
peroxide,  with  the  production  of  much  carbon  dioxide,  which, 
being  carried  in  the  percolating  waters,  might  be  a sufficiently 
powerful  agent  to  bring  about  the  alteration  of  the  granite.  This 


ON  A BASIC  ROCK  DERIVED  FROM  GRANITE.  677 


explanation  would  account  for  the  alteration  of  gneiss  as  well  as 
of  granite,  but  does  not  remove  the  difficulty  afforded  by  the 
lack  of  metamorphism  in  the  iron  ore  at  the  Sterling  mine.  There 
are,  moreover,  other  facts  which  need  not  be  discussed  here,  indi- 
cating that  the  ore  is  probably  a secondary  concentration  younger 
than  the  granite.  Upon  this  supposition  is  based  the  second, 
and,  in  the  writer’s  opinion,  more  probable  hypothesis  to  account 
for  the  alteration  of  the  granite. 

This  hypothesis  assumes  that  at  the  time  of  the  granitic  intru- 
sion the  ore  had  not  been  formed,  its  present  place  being  occu- 
pied by  other  rocks,  chiefly  limestone.  The  ore  was  formed  by 
the  gradual  replacement  of  the  limestone,  through  the  agency  of 
solutions  which  at  the  same  time  produced  the  alteration  of  the 
granite.  This  explanation  requires  that  a source  shall  be  found  for 
the  solutions  supposed  to  bring  about  the  whole  series  of  changes. 
It  is  believed  that  such  a source  exists  in  a ridge  of  gneiss  which 
rises  a few  rods  west  of  the  mine.  The  rock  of  this  ridge  is 
highly  pyritiferous  and  contains  also  much  magnetite.  As  the 
result  of  weathering,  the  surface  becomes  rusty,  and  the  pyrite 
almost  wholly  disappears,  leaving  the  rock  light  and  porous. 
While  this  pyritiferous  rock  is  not  shown  directly  at  the  mine, 
there  can  be  no  doubt  of  its  presence,  as  its  strike  is  such  as  to 
carry  it  very  close  to  the  ore  body. 

The  oxidation  of  the  pyrite  yields  solutions  containing  iron 
sulphates  and  sulphuric  acid.  These  solutions  must  be  capable 
of  producing  very  marked  chemical  effects,  and  are  just  the  sort 
of  agent  required  to  account  for  all  the  phenomena  under  consid- 
eration. Working  down  the  dip  and  coming  in  contact  with 
limestone  and  granite,  they  would  change  the  former  to  an  iron 
ore,  with  the  consequent  formation  of  solutions  of  lime  and  mag- 
nesia sulphates.  These  solutions,  as  well  as  those  derived  directly 
from  the  pyrite,  would  attack  the  granite,  and,  being  very  differ- 
ent from  the  common  agents  of  alteration,  the  product  of  their 
action- would  naturally  be  of  an  unusual  character,  as  is  the  case 
with  the  rock  under  consideration.  The  solutions  would  have 
much  more  powerful  chemical  action  than  the  ordinary  agents  of 


678 


THE  JOURNAL  OF  GEOLOGY. 


weathering  and  would  supply  the  elements  which,  as  analysis 
shows,  have  been  added  to  the  rocks. 

While  it  is  impossible  to  trace  this  process  with  precision, 
and  there  is  no  positive  proof  that  it  is  what  has  actually  occurred, 
still  the  explanation  has  much  to  commend  it.  It  accounts  for 
the  association  of  the  altered  granite  with  the  ore,  and  its 
absence  elsewhere  in  the  region,  by  assigning  the  alteration  of 
the  granite  and  the  formation  of  the  ore  to  a common  cause. 
It  explains  the  very  unusual  character  of  the  alteration  as  the 
result  of  an  unusual  agent.  Evidence  of  a general  nature  bear- 
ing in  the  same  direction  is  afforded  by  the  fact,  stated  by 
Roth,1,  that  several  of  the  hydrated  silicates  of  iron  and  mag- 
nesia, to  which  the  green  aggregate  of  the  altered  granite  is 
quite  similar,  are  formed  where  the  products  of  the  weathering 
of  pyrite  act  upon  silicates.  These  facts  seem  sufficient  to  war- 
rant the  tentative  acceptance  of  the  hypothesis  as  a reasonable 
explanation  of  the  phenomena  observed. 

Similar  rocks  elsewhere  in  the  regio?i. — As  previously  stated, 
the  ore  at  all  the  mines  of  the  region  is  associated  with  rocks 
more  or  less  similar  to  that  at  the  Old  Sterling,  and  generally 
called  serpentine.  That  these  rocks  have  been  subjected  to  a 
process  of  alteration  analogous  to  that  of  the  granite  has  already 
been  suggested  as  probable,  but  it  has  not  been  found  possible 
always  to  determine  their  original  character.  At  the  Dixon 
mine  the  “ serpentine  ” is  plainly  an  altered  granite,  like  that  of 
the  Old  Sterling.  The  “ serpentine  ” at  the  Clark  and  Pike 
mines  is  an  altered  gneiss,  but  at  the  Caledonia  mines  its  nature 
is  somewhat  uncertain.  Here  it  is  an  aphanitic  mass  showing, 
as  a rule,  no  trace  of  its  original  minerals  or  structure.  By  Shep- 
ard2 it  was  included  in  his  mineral  species  Dysyntribite,  whose 
variable  nature  was  afterwards  shown  by  Smith  and  Brush.3  Its 
relation  to  the  ore  is  such  as  to  suggest  an  intrusion,  but  the 

XJ.  Roth,  Allgemeine  und  Chemische  Geologie,  Vol.  I.,  p.  238. 

2 R.  U.  Shepard,  Am.  Jour.  Sci.  (2)  XII.,  p.  209.  Treatise  on  Mineralogy, 
p.  146. 

3J.  L,  Smith  and  G.  J.  Brush,  Am.  Jour.  Sci.  (2)  XVI.,  p.  50. 


ON  A BASIC  ROCK  DERIVED  FROM  GRANITE.  679 

data  are  scanty  and  unreliable.  The  composition  of  the  rock, 
shown  in  IV.,  is  so  different  from  that  of  the  Old  Sterling  rock, 
as  to  raise  some  doubt  of  a unity  of  origin.  This  is  particu- 
larly true  when  it  is  considered  that  the  Caledonia  rock,  if 
derived  from  a granite,  has  generally  lost  all  trace  of  its  quartz, 
and  yet  has  suffered  less  chemical  change  than  has  the  Old 
Sterling  rock,  in  which  much  quartz  still  remains.  This  fact 
might,  however,  be  accounted  for  by  some  difference  in  the 
solutions  causing  the  alteration,  or  by  a more  complete  crushing 
of  the  granite.  The  latter  explanation  is  particularly  probable, 
as  the  granite  of  the  region  not  uncommonly  runs  over  into  very 
fine  granulitic  phases.  There  are,  moreover,  very  pronounced 
indications  of  crushing  and  shearing  in  the  rock  of  this  locality. 
Some  specimens  of  the  rock,  however,  contain  quartz,  and  in 
thin  sections  closely  resemble  the  Old  Sterling  specimens. 
Examination  of  these  sections  makes  it  difficult  to  avoid  the 
conclusion  that  the  Caledonia  “ serpentine  ” is  also  an  altered 
granite.  The  microscopical  evidence  in  favor  of  such  a conclu- 
sion is  very  strong,  though  not  affording,  as  in  the  case  of  the 
Sterling  rock,  a complete  demonstration. 

From  the  facts  at  hand  it  may  be  stated  that  the  so-called 
serpentine  of  the  various  mines  is  derived  from  different  rocks, 
whose  character  must  be  determined  in  each  case.  There  is 
nothing  to  indicate  that  the  original  rock  was,  in  any  instance, 
a basic  intrusion,  but,  on  the  contrary,  where  it  has  been  found, 
it  is  decidedly  acid.  Moreover,  the  alteration  products  are  not 
sufficiently  uniform  in  character  to  be  grouped  under  a specific 
name,  and,  even  were  this  done,  the  term  serpentine,  which  has 
always  been  applied  to  them,  would  have  to  be  supplanted  by 
something  more  in  accord  with  their  composition. 

C.  H.  Smyth,  Jr. 

Hamilton  College, 

Clinton,  N.  Y. 


